Idiopathic hypertrophic subaortic stenosis (IHSS) is a relatively common disorder characterized by cardiac hypertrophy, classically affecting the interventricular septum to a predominant extent. A pressure gradient is frequently present or provocable across the left ventricular outflow tract. This gradient represents obstruction from septal thickening and an abnormal systolic anterior motion of the mitral valve, or SAM, toward the septum. Because of the central role of SAM in the generation of obstruction, it is important to understand its mechanism. The most widely held theory relates SAM to the acceleration of flow in the narrow outflow tract, causing pressure to fall above the leaflet. However, this theory fails to explain several important features of SAM, particularly how it can occur when the papillary muscles (PMs) are tethering the mitral valve posteriorly and at a time early in systole when Venturi forces are likely to be weak. An alternative mechanism for the development of SAM is proposed, combining flow-related forces with leaflet/chordal slack resulting from the anterior and inward displacements of the PMs in IHSS. Anterior PM shifts could also buckle the leaflet directly into the path of flow, so that drag forces would drive SAM. In order to test this PM hypothesis, models of septal thickening and PM displacement have been developed. Further work with them needs to be done to determine whether PM shifts alone can cause SAM, and to what extent they contribute to SAM and obstruction in the presence of septal hypertrophy. The PM hypothesis will be explored both in vivo and in vitro, where critical variables such as PM tension can be more precisely controlled and the flow patterns leading to SAM can be more precisely delineated. The knowledge to be gained from these studies could help us tailor therapy for this complex and vexing clinical problem.